A scanning electron microscope (SEM) includes a SEM column, a chuck for supporting an inspected object, a mechanical stage for moving the chuck and an interferometer. The interferometer includes a controller, a pair of laser heads, a pair of chuck interface mirrors, and a pair of SEM column mirrors.
The pair of SEM column mirrors are connected to the SEM column. The pair of chuck interface mirrors are mechanically coupled to the mechanical stage via mechanical couplers.
The pair of laser heads illuminate the pair of chuck interface mirrors and the pair of SEM column mirrors with laser beams and receive reflected laser beams. The interferometer determines, based on the reflected laser beams, the X-axis distance and the Y-axis distance between the pair of chuck interface mirrors and the pair of SEM column mirrors.
One or more of the SEM column, the mechanical stage, the mechanical couplers and the chuck interface tend to heat during the operation of the SEM. The heating causes the SEM column, the mechanical stage, the mechanical couplers and the chuck interface to expand.
There are differences in the thermal expansion coefficients of the SEM column, the mechanical stage, the mechanical couplers and the chuck interface. The differences between these thermal expansion coefficients introduce an X-axis distance error and a Y-axis distance error.
The X-axis distance error and the Y-axis distance error represent the difference between the displacements, due to the heat, of the pair of chuck interface mirrors and the pair of SEM column mirrors.
The SEM column illuminates the inspected object with a primary beams of electrons. The X-axis distance error and the Y-axis distance error may cause the interferometer to perform false location corrections of the primary beam.
There is a growing need to provide a device for reducing the X-axis distance error and the Y-axis distance error.